Recommended free space on a VMFS3 volume

[arisaperstein]

I have a VMFS3 partition of 2.11TB. We are hosting 120 vm sessions on a 5-node ESX 3.0.1 cluster. We are snapshotting all the sessions in order to do vmdk backups. If we were to snapshot all the sessions at the same time, how much free disk space would I need to accomplish this? I had calculated approx 18MB per VM therefore I would need 2.4GB of free space.

Any thoughts?

Thanks.

[esiebert7625]

How long do you plan to keep the snaps? What types of servers? How big are your VM drives? I would say in general if you keep them for less then a day and there are not a huge amount of changes to the disk after the snap for a 20GB VM HD you would need about 2-3GB.

Also see this post, don't forget to allow room for your vswp files. By default the vswp files will consume the same amount of disk space as the RAM assigned to the VM.

Sizing LUN for enough free space after VM - http://www.vmware.com/community/thread.jspa?messageID=649338

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Thanks, Eric

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[TiBoReR]

I already saw in a VMworld document that you should keep 20% of all the space you use for your VM to do your snapshots.

So if you have 120VM of 20GB each = 2400GB

So you should have 480GB of free space unless you are sure they will not be snapshot at the same time and they will be snapshot only the time of a backup.

You also need to keep free space for vswap. It's the same size of the RAM you allocate to each VM.

So if you have 120 VM with 1GB of RAM each, you need 120GB.

[esiebert7625]

20% is a little high for the short period of time that the snapshot will be out there while it's backed up. Again it depends on the time and type of server. Database/Exchange servers will grow alot faster then web servers.

You can decrease the space needed for vswp files by creating memory reservations. I usually create them for half the memory installed in a VM.

[arisaperstein]

To clarify, I am not struggling with how much space I need for each VM. That much I know and have allocated. The question really is, how much do I fill the disk up or how much of the datastore do I need to leave free to allow all 120 sessions to be snapped at the same time. Assume that the average vmdk size is 16GB and when we snap we are not snapping with memory state. The snapshots will remain for a few hours and then be unsnapped by the very same process that snapped them in the first place. We also have a safeguard in place that unsnaps any snaps that may have remained snapped from the previous week.

[esiebert7625 · Accepted solution]

You will probably never want to go below 50GB free on a volume of that size with that many VM's. You do not want any chance of running out of disk space when operating with snapshots. Also if you try and snap all 120 VM's at once you could bring your ESX server to it's knees because of SCSI reservations.

For 120 VM's with 16GB each for a few hours and assuming a moderate rate of disk modifications during the snapshot time I would say around

180-200GB.

[depping]

I always add 10% to the total if the snaps are only used for VCB, should me more than enough.